It was very kind of you to help her. Particularly as she appears to have what is tantamount to a disability.
Sometimes when all is going well, I am eager to be charitable. I have learned that I need to carefully evaluate before I say yes to something (loaning at item, offering my time, etc). If I suspect that I'm going to end up griping about my good deed, I say no, since I consider it a worse fault in myself to pretend charity than to refuse it.
So if'n I were you, I would only lend the car if I was ready to accept damage to it (which pretty much means no, then). There's no point in lecturing the woman. She's obviously missing essential social skills and she won't actually profit from the explanation.
So I'd say "No, I'm afraid I can't lend you the car" without any further explanation. Repeat ad nauseum. The answer to "why" is a pleasant "I'm not lending it any more."
And thanks again for having been a compassionate neighbor to someone in need.